Cartridge heater and method of making same



A ril 22, 1958 G. B. DESLOGE 2,831,951

CARTRIDGE HEATER AND METHOD OF MAKING SAME Filed July 6, 1954 Z Ww m wa mi m M5 6 F a United States Patent() CARTRIDGE HEATER AND METHOD OF MAKING SAME George B. Desloge, University City, Mo., assignor to Watlow Electric Manufacturing Company, St. Louis, Mo., a corporation of Missouri Application July 6, 1954, Serial No. 441,269

6 Claims. (Cl. 201-67) W into a casing. In some cases, the space between the casing and core is loosely filled with granular magnesium oxide. This results in poor heat transfer since the heat must pass from the coil through a porous and poor heat conducting cement and/or a loose filler to the casing, which is relatively far from the coil.

One of the principal objects of the present invention is to provide a compressed cartridge heater capable of producing higher operating temperatures without increasing the size of the heater.

Another object of this invention is to provide a cartridge heater having parts made from certain materials and shaped so that the heater can be assembled by swaging or rolling. More specifically, it isan object to provide a special core and lava end plugs which allow the heater assembly to be swaged, thereby compressing the insulating material and improving the thermal conductivity between the inside and the outside of the cartridge heater.

A still further object of this invention is to provide a cartridge heater wherein relatively rigid lead wires are firmly anchored and connected to the resistor without requiring soldering, thereby eliminating the possibility of a faulty or broken connection within the heater.

Another object is to provide a dense and hard heater construction which is not easily damaged. Another object is to provide a sealed heater construction having no openings for grain or other filling material to leak out.

These and other objects and advantages will become apparent hereinafter. I

The invention comprises a cartridge heater having a core with openings for lead wires, a resistor wound on said core and having end portions contacting the lead wires within the openings, and a casing enclosing the, resistor and core with insulating material therebetween. The invention also comprises a method of making a cartridge heater in which the aforementioned parts are assembled and swaged.

The invention also consists in the process and in the parts, elements and arrangements of parts hereinafter described and claimed. In the accompanying drawings which form a part of this specification and wherein like numerals refer to like parts wherever they-occur:

Fig. l is an exploded view of the parts comprising the cartridge heater embodying the present invention,

Fig. 2 is a longitudinal vertical cross sectional of the assembled cartridge heater, 7

Fig. 3 is a transverse cross sectional view of the asview sembled cartridge heater taken along the line 3.3 of

Fig. 2, t

Fig. 4 is anenlarged cross sectional view showing the assembled relationship of the heating elements, and

Patented Apr. 22, 1958 2 is positioned in a casing or sheath 4 with insulating material '5 therebetween to prevent the contact of the heating element with said sheath 4. The insulating material should be preferably magnesium oxide or another granular insulating material having good thermal conductivity and high electrical insulation properties at high temperatures.

The heatingelement comprises a helical resistance wire 6 formed from a straight high resistance wire which is wound in turns about the periphery of the core 2. Lead pins or wires 7, which may be relatively rigid or flexible are inserted in the openings 3 in the core 2. .The ends 8 ofthe resistance coil 6 are directed inwardly along slots 9 provided in each end of the core 2, and are directed into the longitudinal openings 3 at opposite ends of the core, one end being positioned in each opening. The lead pins 7 are inserted into the openings 3 so that each pin extends through the core 2 to the other end thereof, and is positioned in contacting relation through out the entire length of the end 8 of the coil 6 in each opening 3. If desired, the lead pins 7 can be slightly smaller in diameter than the openings 3 so as to substantially fill said openings, and may be provided with grooves for receiving the ends 8 of the coil 6.

The core 2 is made preferably of a ceramic material which is capable of being made to close tolerances and which is capable of being compressed or swaged. After compression, the core is pulverized and assumes a new straight and perfectly round cylinders with no warping and has good insulation properties at high temperatures.

The casing 4, which circumscribes the heater elements, is preferably made of. metal so as'to distribute the heat created in the coil evenly throughout the surface of theheater. The lead pins 7 extend from one end of the casing 4 and are held in place by an end plug 10 formed of lava. A bushing 11 also formed of lava is provided at the other end to insulate the heating elements from an end plate 12 which closes the endof the casing 4. The helical coil 6 is insulated from the sheath 4 axially by an insulating material 5 such as granular magnesium oxide.

The lava used for the end plugs 10 and 11 should be capable of being compressed or swaged without breaking or crumbling. A partially fired hydrus aluminum silicate, known in the trade as a GradeA lava, gives satisfactory results. Such a material remains soft even with little or'no moisture therein. Grade A lava, which is sold by the American Lava Corporation, gives up its water of crystallizationand gets very hard at about two thousand degrees Fahrenheit. However, Grade A can be expanded and dried without getting hard. It is preferable to use partially fired, dried, Grade A lava having little or no moisture therein.

The process of assembling the cartridge heater 1 is as follows: The resistance wire is wound onto the core 2,

' end plug 10 and an end'disk 13 are placed on the pins 7. The end disk 13 can be made from any soft material,

such as fiber or plastic, etc., which is capable of being removed. This assembly is inserted into the sheath 4, which is then filled with the insulating material 5. The open .end of sheath 4 is .closed by .the insertion ofthe lava bushing 11 and a fiber or plastic bushing 14. At this point in manufacture, the elements .of the cartridge heater 1 are loosely held togetherin their respective positions, the fiber end disk 13 and the fiber bushing 14 enclosing the assembly and maintaining proper positioning of the elements for swaging.

The assembly is then swaged, and in so doing, the lava end plugltl is compressed and fits tightly around the pins sheath 4. The core 2 is compressed so that the lead pins 7 firmly contact the ends 8 of the resistor and are firmly secured in the core.

The sheath 4 is then trimmed back on both ends and the fiber disk 13 and fiber bushing 14 are removed. The end plate 12 is inserted in the sheath 4 in the place left by the fiber bushing and secured to the sheath 4 by suitable means, such as a weld. If desired, the end of the casing may be rolled over. Although the lava bushing 11 is an adequate plug for the end of sheath 4, the end plate 12 is provided for better thermal conductivity and to close the end of the metal casing.

The hereinbefore described construction and method have important advantages over known constructions and methods. By using lead pins in the manner described, the leads cannot be pulled .out or broken and outside connections to lead wires can be made. This allows the lead connections to be repaired. In addition, there are no soldered connections within the casing, so that there is little likelihood :that defective heaters will be produced initially or that the lead conduit will be pulled from the heater thereby breaking the soldered connection. The long wound core provides for more uniform heat and allows the coil to be-positioned closer to'the casing. This makes the temperature difference between the casing and the coil smaller, gives the heater longerlife, and permits higher operating temperatures, even up to 2000 F. The sealed ends of the heater prevent the granular material from shaking out and prevent foreign matter from getting into the casing. The swaged or rolled construction results in a denser and more uniform unit which is less likely to be damaged or deformed and which has better thermal conductivity.

The present construction can be used to make relatively long cartridge heaters heretofore difficult to make because of the requirement of a relatively long core member. Long cartridge heaters can be made using the present invention by placing two or more cores 2 in endwise abutting relation with a circular 'insulator'therebetween, said circular insulator having two holes for receiving the lead pins therethrough. The circular insulator is preferably larger in diameter than the core, being preferably the same as the inside diameter of the casing, to prevent shifting. Each core assembly is independent of the other and has the ends of the resistor within its own openings so that the insertion .of the lead pins through two or more endwise abuttingv cores .2 prior to swaging automatically produces a long cartridge heater having a plurality of heaters therein arranged in parallel. Thus, should one unit become defective for any reason, the other units will not .be affected since parallel circuits are produced by inserting long lead pins through a plurality of separate core assemblies.

This invention is intended to .cover all changes and modifications of .the example of the invention herein chosen, for the purposes of the disclosure, which do notconstitute departures from the spirit and scope of the invention.

What I claim is:

1. In a method of assembling a cartridge heater, the steps including winding a wire resistor upon a core having openings therethrough and positioning the ends of the resistor linearly in said openings, inserting lead wires into said openings in substantially parallel linear contacting relation withthe ends of said resistor, placing an insulating bushing over said lead wires thereby forming a core assembly, .insertingsaid core assembly into a casing, filling said casing with an insulating material, closing the end of said casing with a plug and swaging the elements into assembled relation.

2. A method of assembling a cartridge heater which comprises winding a wire resistor upon a crushable ceramic core having two longitudinal openings therein extending substantially throughout the length thereof, positioning one end of said resistor in one .of said openings and the other .end .in .the other of said openi38 inserting a lead wire into each of said openings so that said lead Wires contact the ends of said resistor and substantially extend throughout the length of said openings, placing .an insulating bushing of crushable lava over said lead wires thereby forming a prepared core assembly,

inserting said prepared core assembly into a .metal casing of slightly larger diameter, filling the space between said core assembly and .said casing with magnesium oxide so as to insulate said resistor from said casing, closing theother end of said casing with a plug, and swaging the elements into assembled relation so that said lead wires are held in fixed positionby said lava bushing.

3. A method of assembling a cartridge heater which comprises winding a high resistance wire upon a crushable ceramic core to form a helical resistor, said core having two longitudinal openings therein extending substantially throughout the length thereof, positioning one end of the resistor in one .of said openings, and positioning the other end'of said resistor in an opposing direction in the other of .said openings, inserting lead wires into said openings so that said lead wires extend substantially throughout the length of said openings in said core and are positioned in contacting relation with the ends of said resistor, placing an insulating bushing of lava and a bushing of fiber over said lead wires to form a prepared core assembly, inserting the prepared core into a metal casing ofs'lightly larger diameter, filling'the space between said coreassembly and said casing with granular magnesium oxide so as to insulate said resistor from said casing closing the end of said casing with an insulating lava bushing, swaging the elements into assembled relation so that the lead wires are compacted and a connection is formed between thelead wires and the ends ofsaid resistor, removing the fiber bushing, and securing a metal disk to the end of-the heater opposite said lead wires.

4. A heater comprising an insulting core having openings therein extending substantially throughout the length thereof, a helical resistor wound on said core with end portions positioned in said openings, said resistor having one end portion extending into one of said longitudinal openings in one direction and another end portion extending in the opposite direction in the other of said longitudinal openings, lead'members having end portions positioned insa'id openings in substantially linear contacting relation with the'end portions of said resistor, said lead members each having one end which terminates within the heater, a casing enclosing said resistor wound core, insulating material "between said casing and said core, and plugs closingthe end portions of said casing.

5. A cartridge heater comprising a. cylindrical ceramic insulating core of uniform cross-section having at least two longitudinal openings therein of uniform cross-section extending substantially throughout the length thereof, said openings having walls substantially parallel to the axis of said core, a wire wound on said core to form a helical resistor, said wire having one end portion extending into one of said longitudinal openings and the other end portion extending in the opposite direction into the other of said longitudinal openings, a lead wire having an end portion of uniform cross-section positioned in each of said longitudinal openings in substantially parallel linear contacting relation with the end portion of said resistor, each of said lead wires having one end which terminates within the heater, a casing circumscribing said resistor wound core, insulating material between said core and said casing, and compressed plugs closing the ends of said casing.

6. A cartridge heater comprising a cylindrical ceramic compressible insulating core of uniform diameter having at least two longitudinal openings therein extending substantially throughout the length thereof, said openings having walls parallel to the axis of said core, a high resistance wire wound on said core to form a helical resistor, said wire having one end portion extending into one of said longitudinal openings and the other end portion extending in the opposite direction into the other of said longitudinal openings, a lead wire having an end portion of uniform cross-section positioned in each of said longitudinal openings substantially throughout the length thereof in substantially linear contacting relation with the end portion of said resistor, a metal casing'circumscribing said resistor wound core in close proximity thereto, compressible granular insulating material between said core and said casing, compressible plugs closing the ends of said casing, said lead wires extendingthrough only one of said plugs, said elements being swaged into final assembled relation so that the helical resistor is in close proximity to said casing and is held in fixed position by said compressed insulating materials.

References Cited in the file of this patent UNITED STATES PATENTS 1,525,831 Steiner et a1. Feb. 10, 1925 1,708,961 Abbott Apr. 16, 1929 1,735,831 Lightfoot Nov. 12, 1929 2,030,460 Morton Feb. 11, 1936 2,490,934 Vogel Dec. 13, 1949 FOREIGN PATENTS 256,908 Great Britain Aug. 19, 1926 

